Asthma is a complex disease associated with bronchial hyperreactivity and atopy, making asthma a disease with a phenotype that has been clinically difficult to define. Despite intense research, prevalence of asthma remain relatively high. The key reason for the high prevalence and morbility is that the fundamental mechanisms predisposing individuals to the development of asthma are not understood. Familial aggregation observed in this pathology has prompted for the search of an involved genetic component. This task is difficult due to the complex nature of asthma. A universally accepted definition for this disease is not available, clinical expression can be modulated by environmental factors, and inheritance does not follow a clear Mendelian pattern. Establishment of more precise clinical and laboratory criteria has improved the design and interpretation of genetic studies. Twin analysis and segregation studies have demonstrated an important genetic component with a probably multifactorial pattern of inheritance. "Sib pair" studies and familial segregation analyses have shown linkage between some chromosomal regions and asthma, including chromosome 5, 6, 7, 11 and 14. The search for major genes in these chromosomal segments has been focused on loci involved in the allergic process. Among these, the loci for IL-9 and IL-13 in chromosome 5 seem to play an important role in the pathogenesis of asthma. Understanding the fundamental gene-environmental interactions in the development of asthma should lead to earlier identification of susceptible individuals and more effective approaches for disease prevention.